1. Field of the Invention
This invention relates to a matrix circuit of an FM stereo multiplex demodulation circuit which is formed in an integrated circuit configuration and is used in a device capable of receiving FM stereo broadcast, and more particularly to a circuit for shunting a demodulated composite signal current in the preset ratio.
2. Description of the Related Art
A matrix circuit of an FM stereo multiplex demodulation circuit of the prior art is constructed as shown in FIG. 5. The block diagram thereof is shown in FIG. 7. That is, in the matrix circuit of FIG. 5, Q51 and Q52 denote NPN transistors whose emitters are connected together via respective matrix resistors R.sub.1 and the connection node therebetween is connected to a ground terminal via a matrix resistor R.sub.2. The bases of the NPN transistors Q51 and Q52 are connected to an input node 50 and a bias voltage (V.sub.1) source, respectively. When an FM-demodulated composite signal voltage .DELTA.vi is input to the input signal node 50, the input signal .DELTA.vi is divided into shunt currents .DELTA.i.sub.1 and .DELTA.i.sub.2 which flow as collector currents of the NPN transistors Q51 and Q52 in the preset ratio.
In FIG. 7, 11 denotes a buffer circuit, 12 a V/I converter, 13 a tone controller, 14 an I/V converter, 15 a matrix circuit, and 16 a switching decoder.
Shunt currents .DELTA.i.sub.1 and .DELTA.i.sub.2 output from the matrix circuit are supplied to a switching decoder SD, switching-controlled by complementary switching pulse signals of 38 KHz and output as a left-channel signal Lout and a right-channel signal Rout.
In the above operation, the input signal .DELTA.vi, shunt currents .DELTA.i.sub.1 and .DELTA.i.sub.2, matrix resistances R.sub.1 and R.sub.2 have the following relations: ##EQU1##
In order to set the separation between the left-channel signal Lout and the right-channel signal Rout to a maximum value, the resistances R.sub.1 and R.sub.2 are determined to satisfy the following expression: ##EQU2##
As shown in FIG. 7, since the matrix circuit of the prior art receives a voltage signal .DELTA.vi as an input, an FM-demodulated composite signal cannot be directly input in a case where the FM-demodulated composite signal is a current output, and in this case, the FM-demodulated composite signal can be input only after the composite signal is converted into a voltage signal .DELTA.vi by means of a current-voltage (I/V) converting circuit (14 in FIG. 7).
As described above, the matrix circuit of the prior art cannot directly receive an FM-demodulated composite signal as an input in a case where the FM-demodulated composite signal is a current output, and can receive the same only after it is converted into a voltage signal .DELTA.vi by use of the I/V converting circuit. Therefore, it tends to be restricted by the internal dynamic range, the operation efficiency of the power source is lowered and the number of elements used becomes larger.